Over recent years, the greenhouse effect due to carbon dioxide (CO2) has been pointed out as one of the causes for the global warming phenomena, and there has been an internationally urgent need to take countermeasures there against to protect the global environment. Generation sources of CO2 extend to every activity field of human beings where fossil fuels are burnt, and discharge regulations of CO2 tend to be further enhanced in years to come. With this trend, for power generators such as thermal power stations and the like using a large amount of fossil fuels, CO2 removal methods such as a PSA (pressure swing) method, a membrane separation method, reaction absorption using a basic compound, and the like have been researched.
Of these, as an example of the CO2 removal method using a basic compound, Patent Document 1 proposes a decarbonation method using an amine compound aqueous solution as a CO2 absorbent. In this method, since CO2 and an amine compound exothermally react with each other, an absorbing solution temperature in a CO2 absorption section increases, resulting in an increase in a vapor pressure of the amine compound. In other words, since the temperature increase causes a large amount of the absorbing solution to evaporate, the amount of the amine compound accompanying a decarbonated gas increases. Therefore, an absorption column is provided with a washing section, and in this washing section, a decarbonated gas is brought into vapor-liquid contact with a part of reflux water, as washing water, from a cooler disposed on the downstream side of a desorption column for an amine compound aqueous solution, whereby the amine compound accompanying the decarbonated gas has been recovered into a liquid phase. Further, to enhance amine recovery efficiency, Patent Document 2 proposes a method employing a multistage structure for the washing section.
FIG. 2 illustrates one example of such a conventional CO2 removal apparatus. This apparatus includes a decarbonator 1 for removing carbon dioxide (CO2) contained in a combustion exhaust gas in a contact section (a packed section) 2 that makes counterflow contact of the combustion exhaust gas and an amine compound aqueous solution; a desorption column 13 for regenerating an amine compound aqueous solution by heating the amine compound aqueous solution discharged from the decarbonator 1; and means (a reflux water supply line) 21 for supplying reflux water from a cooler 19 disposed on the downstream of the desorption column 13 to the decarbonator 1. Herein, in the figure, the symbols 3, 4, and 5 represent a combustion exhaust gas supply line, a CO2-removed combustion exhaust gas, and an amine compound aqueous solution supply line, respectively. The symbols 6, 9, 14, and 20 each represent a nozzle. The symbols 7, 8, and 10 represent a water circulation pump, a cooler, and a CO2-absorbed amine compound drawing line, respectively. The symbols 11 and 12 represent a gas to be treated and a blower, respectively. The symbols 15, 16, 17, and 18 represent a lower packed section of the desorption column, a pump, a CO2 separator, and discharged CO2, respectively. The symbols 22 and 23 represent a heat exchanger and a reboiler, respectively. And, the symbols 24 and 25 represent a washing section and an upper packed section of the desorption column, respectively.